1. Field of the Invention
The present invention relates to semiconductor laser apparatus.
2. Description of the Prior Art
Semiconductor laser devices are used in a wide variety of fields because they are compact in size, have a relatively higher efficiency and are capable of producing modulated beams when current is suitably controlled. But it is well known in the art that the intensity of output light from the semiconductor laser device varies over a wide range due to the environmental temperature, ageing of the semiconductor laser device, etc.
In order to maintain the output light intensity constant, there has been proposed a method wherein the intensity of the laser beam emitted from the semiconductor laser device is detected by an intensity sensor, and in response to the output from the intensity sensor a drive circuit controls the current applied to the semiconductor laser in such a manner that the intensity of the laser beam may be maintained constant. However the temperature of the p-n junction (to be referred to as "the junction" hereinafter in this specification) remarkably increases because the density of current flowing through the junction increases to an extreme degree.
Meanwhile the junction temperature vs. output light intensity characteristic curves with the drive current I of the semiconductor laser device as a parameter shown in FIG. 1 are well known in the art. That is, with increase in temperature at the junction, the output light intensity decreases. Therefore when one tries to maintain the output light intensity constant, the output light intensity decreases with increase in temperature at the junction. When current is increased, the output light intensity is increased to an initial level. As described above, however, the increase in current causes the junction temperature to rise, thus resulting in the thermal runaway cycle of the temperature rise, the current increase, the temperature rise, and so on. As a result, stable and reliable operation of the semiconductor laser device cannot be ensured.
As an output light intensity sensor, a semiconductor photoelectric converter may be used because it is convenient to use. Its output, however, depends upon its surrounding temperature so that the control of the semiconductor laser device with a higher degree of accuracy is not accomplished. The sensitivity of the semiconductor output light intensity sensor widely varies depending on its surrounding temperature especially in infrared wavelengths, which are emitted by most semiconductor lasers. Therefore, it is essential that the output light intensity sensor must be also maintained at a constant temperature so that the semiconductor laser device will be controlled with a desired degree of accuracy.